Since a liquid crystal screen of a liquid crystal display (LCD) is not self-emissive, a mirror is placed behind liquid crystals to reflect light which enters the screen through a front side of the liquid crystal display, or a backlight unit (BLU) is placed at a rear side of the liquid crystal display to emit light such that transmittance and colors of the light are adjusted to allow the screen to be viewed.
FIG. 1 is a sectional view of a typical backlight unit. Referring to FIG. 1, the backlight unit may include a light source 10, a light guide plate 20, a reflective sheet 30, and optical sheets 40, which include a diffusive sheet 42, a prism sheet 44, a protective sheet 46, and the like. As for the light source 10, a self-emissive device such as a light emitting diode (LED), fluorescent lamp, cold cathode tube, laser diode, organic EL, and the like is used.
Light emitted from the light source 10 is incident upon an incident face of the light guide plate 20. The light guide plate 20 serves to convert spot light into surface light, and may include a wedge-shaped light guide plate, one surface of which has an oblique plane, a planar light guide plate, and the like. Light emitted from the light source 10 is converted into face light having uniform brightness through total reflection, diffusive reflection, refraction and diffraction inside the light guide plate 20, and exits the light guide plate through an upper surface (front surface) and a lower surface (rear surface) thereof.
Here, the light that exits the light guide plate 20 through the lower surface thereof is incident upon the reflective sheet 30. The reflective sheet 30 reflects the light towards the upper surface of the light guide plate 20. Then, the light reflected towards the upper surface of the light guide plate 20 enters the optical sheets 40. The optical sheets 40 are generally composed of the diffusive sheet 42, the prism sheet 44, the protective sheet 46, and the like. The diffusive sheet 42 serves to spread the incident light, the prism sheet 44 serves to partially collect the light, and the protective sheet 46 serves to protect the diffusive sheet 42 and the prism sheet 44 from being damaged due to foreign matter, scratches, and the like. Typically, light emitted from the light source increases viewing angle and reduces brightness while passing through the light guide plate 20 and the optical sheets 40.
Conventionally, the light guide plate is generally formed of resins such as polymethyl methacrylate (PMMA) and the like. However, in order to provide displays capable of projecting clearer images and to prevent deformation of the light guide plate due to heat generated from the light source, such resins are being replaced by an aromatic polycarbonate resin composition having high heat resistance. Particularly, since a light guide plate used in a portable display requires high thermal stability and long term reliability under high temperature and high humidity conditions, such a light guide plate is generally formed from an aromatic polycarbonate resin composition.
Recently, with diversification of the size of portable displays such as mobile phones and the like, there is a need for a light guide plate that has a large size and a smaller thickness than existing light guide plates having a size of about 2 to about 4 inches and a thickness of about 0.5 mm. Upon injection molding of such a large and thin light guide plate, it is necessary to secure sufficient flowability and transferability of the aromatic polycarbonate resin by injecting the aromatic polycarbonate resin at a higher temperature than a typical molding temperature. However, the light guide plate formed by injection molding at high temperature can exhibit a yellow color (yellowing phenomenon) in surface diffusion of light. Such a yellowing phenomenon of the light guide plate can become severe in use under high temperature and high humidity conditions over time.
Upon injection molding, additives such as polyol can be added to a polycarbonate resin in order to improve thermal stability, colors, and the like (see Korean Patent Publication No. 2011-0050381A and the like). However, this method can also cause side reactions due to the additives in use under high temperature and high humidity conditions for a long time.
Therefore, there is a need for a light guide plate that can reduce color deviation under high temperature and high humidity conditions while securing excellent properties in terms of discoloration resistance, brightness property (brightness uniformity), and the like.